The present invention relates in general to oscillators and, more particularly, to a relaxation oscillator circuit for providing smooth voltage transitions at the terminals of a charging capacitor which desensitizes the oscillator to the effects of the inherent stray capacitance and improves the accuracy of the output frequency.
Oscillators are used in many well known applications including FM (frequency modulation) demodulators. In FM transmissions, the instantaneous frequency of the FM signal will vary in relation to the information content of the baseband signal which is modulated onto the FM carrier. Within the FM demodulator, the instantaneous frequency of the FM signal is compared to the oscillator frequency which is typically tuned to the carrier or center frequency of the operating spectrum of the FM signal. Generally, the FM demodulator provides an amplitude modulated output signal which is proportional to the deviation of the frequency of the FM signal from the tuned center frequency of the oscillator. Any inaccuracies in the oscillator frequency can directly effect the magnitude of the frequency deviation which may cause misinterpretation of the FM signal. Hence, the accuracy and stability of the relaxation oscillator is critical to the proper operation of the FM demodulator.
Relaxation-type oscillators typically include a capacitor which charges and discharges between trip threshold potentials and a switching circuit which alternates the direction of charge on the capacitor. One such relaxation oscillator uses first and second transistors as the switching circuit having the collectors cross-coupled to the bases, and a floating capacitor which is coupled between the emitters of the transistors. The emitters are commonly connected to a constant current supply. The first and second transistors alternately turn on and off as determined by the charge and discharge rate of the floating capacitor which defines the frequency of the oscillator.
One draw back to the conventional oscillator is the inherent sudden voltage jumps at the terminals of the floating capacitor the instant that the transistors change state. Ideally, these sudden voltage jumps are equal in magnitude; however, the effects of the stray capacitance, principally associated with the terminals of the floating capacitor, cause the voltage jumps to be nonsymmetrical which tends to shorten the discharge times of the floating capacitor thereby increasing the output frequency. In addition, the derivative of the voltage across the capacitor (V.sub.c) with respect to time (dV.sub.c /dt) becomes very large at the transistor switching times. Since the current through the capacitor is proportional to dV.sub.c /dt, the sudden voltage jumps may saturate the collector-emitter conduction path as the transistor turns on. The saturation of the transistors adversely effects the charge and discharge rate of the capacitor, and consequently degrades the accuracy of the oscillator frequency.
Hence, there exist a need for a relaxation oscillator which eliminates the sudden voltage jumps at the terminals of the capacitor which allows the improved oscillator to operate with greater accuracy at higher frequencies.